The purpose of this research is to determine whether the tachycardia from nicotine is vagally mediated. It has been assumed that nicotine is sympathomimetic. our previous results with many other drugs of abuse led to the hypothesis that nicotine, in addition to the other abused drugs that we have studied, produces vagally mediated tachycardia. The subjects were 5 smokers deprived of nicotine overnight and 5 nonsmokers. Eight of the subjects were male, and 2 were female. on separate days, they were given i.v. administrations of placebo (saline) and nicotine (0.75 mg and 1.5 mg). We recorded heart rate continuously before and after injection. The results indicated that the tachycardia from i.v. nicotine was substantial. These increases in heart rate were mirrored precisely by decreases in vagal tone and the 0.10 Hz. rhythm. This pattern of response was consistent with vagal mediation of the effect of nicotine on heart rate. The significance of these results is that nicotine appears not to be sympathomimetic at these doses, as has been assumed previously. In contrast, the autonomic effects of nicotine appear to be mediated parasympathetically.